The present invention relates to a controllable oscillating system, and more particularly, to a controllable oscillating system and related method for generating a differential oscillating signal.
In general, in a radio frequency (RF) communication application, an oscillating circuit (e.g. a transmitting master oscillator or a receiving local oscillator) is required for generating a differential oscillating signal for signal transmission or signal reception. A primary performance criterion of the oscillating circuit is the phase noise characteristic. An oscillating circuit with a lower phase noise means that the oscillating circuit produces less spurious signals outside a desired fundamental frequency signal. The phase noise results from a low frequency noise existing in active elements in the oscillating circuit. For example, active elements in an LC oscillating circuit are primarily transistors included within a cross-coupling driving device in the LC oscillating circuit. This low frequency noise is commonly referred to as flicker noise and up converted to the desired fundamental frequency signal. Thus, the phase noise arises.
FIG. 1 is a schematic diagram of a conventional oscillating circuit 100. As shown in FIG. 1, the oscillating circuit 100 comprises a controllable resonator 105, a cross-coupling driving device 110, and a current source 115 for providing a reference current Iref. The controllable resonator 105 (also called an LC resonator) is formed by capacitors C1 and C2 and inductors L1 and L2. The cross-coupling driving device 110 is formed by NMOS transistors Q1 and Q2 and is utilized for generating a differential oscillating signal composed of oscillating signals S1 and S2. Ideally, only one of the NMOS transistors Q1 and Q2 is turned on at a time. That is, the NMOS transistors Q1 and Q2 should not be conductive simultaneously. However, in practice, both the NMOS transistors Q1 and Q2 may be turned on due to the flicker noise as described above. For simplifying the discussion, it is assumed that only the phase noise resulting from the NMOS transistor Q1 of the cross-coupling driving device 110 is considered and the phase noise could be modeled as a small voltage source Vn at the gate of the NMOS transistor Q1. Please refer to FIG. 2. FIG. 2 shows possible practical voltage waveforms of the oscillating signal S2 and the voltage source Vn shown in FIG. 1. The voltage source Vn resulting from the phase noise may cause the NMOS transistor Q1 to turn on early or late, depending upon the timing when the sum of the voltage source Vn and oscillating signal S2 is higher or lower than a threshold voltage of the NMOS transistor Q1. That is, during each transition time, both the NMOS transistors Q1 and Q2 may be turned on. This problem will become more serious if both the phase noise resulting from the NMOS transistors Q1 and Q2 are considered at the same time.
One of the conventional solutions is to utilize an attenuating device to eliminate the phase noise. The attenuating device is utilized for reducing the feedback gain of the active elements for amplifying signals having frequencies much less than that of the desired fundamental frequency signal. Thus, the amplified low frequency noise can be suppressed. A full description is disclosed in U.S. Pat. No. 6,750,727B1 and U.S. Pat. No. 6,987,425B1, and further details are omitted here for brevity.